A Coarse Fraction Bias in Arabian Sea G Bulloides?

In our recent discussion of the Arabian Sea G Bulloides series, I noticed a remarkable increase in the coarse fraction percentage in the top 1.3 cm of the critical RC2730 core.

Willis has also commented on this. In core 2730, there is a correlation of 0.91 between the fraction of coarse particles and the percentage of G Bulloides.

This raises a couple of interesting questions:

Is there some climatic reason for the increase in the percentage of coarse particles at the top of the core?

Or alternatively, are there possible non-climatic reasons for the increase in coarse particle percentage?

In either event, does this have any implications for how one interprets percentage G Bulloides series? This leads, as so often, into many interesting byways and the answers are not entirely the ones that one would expect.

First, here is my plot showing the increase in percentage G Bulloides, percentage carbonate and percentage coarse fraction for the nearby cores RC2735, RC2730 and 723A. Willis has observed that RC2730 is deeper than RC2735 and that quite different currents may apply. 723A is deeper still. (See here for Willis’s graphs).

Willis observed the following:

As you can see, RC2735 is at an escarpment the edge of an upper plateau, which falls down to a lower plateau where RC2730 is located. Because of this, the coarse fraction will be very sensitive to the exact direction and depth of the currents in the area. If the current is running across the escarpment, there will be a large speed difference over the two areas, which will change the coarse fraction. Alternatively, the presence of the escarpment may mean that the deeper plateau is swept by an entirely different current than the upper plateau.

… probably result from changes in surface productivity associated with monsoon variability. Dissolution at this site may be ultimately controlled by the oxidation of organic matter which appears to be incorporated into the sediments in greater quantity during periods of weak SW monsoon and/or increased dry NE monsoon

However, if monsoon strength were the only reason, we’d expect the two records to move in parallel since the winds will be quite similar at two oceanic sites that are only 10 km apart … but they don’t move in parallel.

Finally, the fact that the increase is all in the top couple of centimetres at least allows for the possibility that the fines have been partially washed out of the top of the sampled core.

Without necessarily disagreeing with any of these observations, I’ve also identified literature that discusses an interesting connection between bioturbation and sediment coarse fraction.

The top few cm of ocean sediments are “mixed layer” in which mixing processes are taking place that are not taking place in sediments that are buried by 30 cm of overlying material. Thus the sediment for 1970 AD obtained by an paleoceanographer 10000 years from now would not be the same as the present sediment – its makeup would be altered by mixing.

McCave (1988), entitled “Biological pumping upwards of the coarse fraction of deep-sea sediments” reports that the coarse fraction in the mixed layer is older than the fine fraction and the difference in age can be substantial. This process was summarized by Sachs et al 2000 as follows:

The impact of bioturbation on radiocarbon dates is discussed in a number of articles by Thomson. Thomson et al 1995 says:

and again:

Thomson et al 1995 showed the following interesting illustration of this effect, in which the mixed layer is dated anomalously old – which they attribute not any errors, but to the higher proportion of older coarse material in the mixed layer, which then makes the mixed layer seem “too” old.

The data and arguments of Wheatcroft (1992) show that entirely different bioturbation intensities apply to the large foraminifera as compared with the bulk finer sediments. The biological diffusivity of the fine (<16 μm) is >1 cm2/yr whereas that for sand-sized particles is < 0.1 cm2/yr. Large foraminifera (>150 μm) have also been shown to be less deeply worked down into a recent turbidite surface than natural radionuclides, in a case where both the foraminifera and radionuclides have been deposited since emplacement 1 ka ago (thompson et al 1988; Thomson and Weaver 1994).

Thomson et al. (1995) concluded that the remaining possibility was particle-size induced differential mixing by benthic organismsa mechanism also used to explain the presence of lag layers of coarse (>2 mm) particles derived from a glacial gravel sediment found in a mud deposit several centimeters above the original horizon of deposition (McCave 1988). In essence, this hypothesis proposes that mixing ceases to be completely random when burrowing organisms encounter large sediment grains, as less energy is required to move such grains upwards than to push them downwards, whilst fine sediment then falls into the void thus created. If this mechanism is applicable to the coarse grained foram fraction of deep-sea sediment, dif\fering rates of transport out of the SML for the fine and coarse fractions will result in an age offset

This could have important implications for the RC2730, RC2735 and 723A results. The highly correlated increase in percentage G Bulloides and percentage coarse fraction both occur in the top 1.2 cm of RC2730 – which would be well within the mixed layer of most sediments. Anderson et al 2002 note in passing for RC2730:

the uppermost four samples, including the replicated 0- to 2-mm sample, were too young (less than zero age), which can be explained by the presence of bomb radiocarbon that has mixed downward over the upper 10 mm

The bioturbation smoothing that would normally affect fine-scale variability is minimal at this site owing to the strong oxygen-minimum zone that exists in the Oman margin.

This assertion seems inconsistent with the prior statement in Anderson et al 2002 (but this may require amendment.)

You can see how the potential knock-on effect of the mixing problem – whether through bioturbation or mechanical means. The G Bulloides series has been extracted from the coarse fraction (plus 150 μm). Suppose there has been upward biological (or other) pumping of the coarse fraction relative to the fines. The offsets in radiocarbon dates in other places can be several hundred years.

We know that the coarse fraction increases dramatically in the top 1 cm of RC2750. What if this represents “biological pumping upward” rather than climatic change? And at this point, we have evidence of the biological pumping upward process but not of a climatic relation between global temperature and coarse fraction at RC2730. Thus at RC2730, where the accumulation rate is only 9 cm/kyr according to the dating in Anderson et al 2002, the coarse fraction in the 0-2 mm sample might be integrating coarse fraction over the past 400 years or so and be a LIA signal as opposed to a 1986 signal.

This is the sort of thing that one would expect specialists to sort out in attempts to replicate the use of percentage G Bulloides as a proxy for temperature. Unfortunately, to date, this hasn’t occurred. This study is a one-off. The author of the only related study – G Bulloides at Cariaco – condemned the use of G Bulloides at his site for large-scale temperature reconstructions (his G Bulloides indicated a warm MWP.)

45 Comments

Am I to understand that the mixing shakes the small flakes down and the big flakes up?
If this is a continous process it kinda messes up the whole chronology, yes? or at least
requires you to average over larger periods to “average out” the mixing

Sort of like the rocks in a garden migrating upwards with the frost. Every year there is a new crop of stones migraitng up from below. The Scientific American had an article on this in the era before it was dumbed down. It is also the reason that the cashews in a can of mixed nuts are always at the top. Agitation of the can acts as the frost does on the rocks.

This is interesting … i’m having a hard time tracking down the various people you cite above though. Where is the Willis paper? And what is the citation for the Anderson et al 2002 paper? I may have missed it somewhere. I hate to be nit-picky, but it would be nice if the references list was complete and in a single format (like they make us do for journals)…it makes tracking down the papers MUCH easier. I’d even rather have a standardized references cited list than links. I understand you may be continuing a discussion from earlier, so everybody knows what you’re referring to, so I apologize for not being in the know…but a complete reference list would be nice. It makes it very difficult for a reader to try and evaluate what you’re talking about.

Without digging into the details yet…the presence and degree of bioturbation is (obviously) highly dependent on oxygen conditions (and history of oxygen conditions)at the site…but, yes, these processes have the potential to wreak havoc on the age model for some archives, no doubt.

In terms of #3 commment, that’s been termed ‘kinetic sieving’ by some…the resulting deposit has inverse grading, but these are from sediment gravity flows (turbidity currents, debris flows, etc.). This is ODP 722?

Bioturbation homogenizes the uppermost few cm of sediment as the worms chew through the mud. It seems reasonable to think that sorting by size might occur. How that happens probably has been characterized experimentally in estuarine waters. To minimize the mixing effects, it’s better to take cores from high sedimentation rate areas and anoxic basins. I’m skeptical about the accuracy of the sample dates in these cores, particularly at the top where the mechanics of sampling can disturb the sediment several cm downcore.

Throughout most of the deep sea, bioturbation severely attenuates centennial-scale variability, but in the Arabian Sea, low-oxygen conditions caused by high productivity prevent burrowing organisms from smoothing centennial-scale events.

Regarding disturbance during sampling, they state:

Each of the box core sediment surfaces was observed to be pristine during core recovery.

So, they do address some of the issues in the short paper, but one would probably want to dig into either supplementary data and/or talk with the researchers themselves.
If we assume there is very minimal to no bioturbation, then another hypothesized mechanism for coarser material at the top might be related to oceanic currents (as Willis suggested, I think).
Anderson et al (2002) state:

Sediments from the Oman Margin are nannofossil-rich foraminifer oozes, produced each summer when strong southwest monsoon winds drive enhanced biological productivity in the northwestern Arabian Sea.

Which, taken at face value, implies that this is not an environment that transports/accumulates coarser material. But, I’m certainly no expert in this regard…there very well could be something erroneous going on with their data.

But “top layer” is always changing, right? every layer was once a top layer and subject to mixing?
So all “layers” have been subject to this mixing at some point, I would think, and i would
expect there to be some variability in deposition rates and mixing rates. I’m way out of my knowledge
comfort zone on this one, so forgive the stupid questions

Throughout most of the deep sea, bioturbation severely attenuates centennial-scale variability, but in the Arabian Sea, low-oxygen conditions caused by high productivity prevent burrowing organisms from smoothing centennial-scale events.

But what is the evidence for this? There is no reference for this assertion. (Of course it’s only in Science.)

There is an oxygen-minimum zone offshore California, but there is thorough bioturbation nonetheless:

The present upper water mass of the northeastern Pacific Ocean off California has a well-developed oxygen minimum zone between 600 and 1200 m wherein concentrations of dissolved oxygen are less than 0.5 mL/L. Even at such low concentrations of dissolved oxygen, benthic burrowing organisms are abundant enough to thoroughly bioturbate the surface and near-surface sediments.

Another reference specifically says that there is bioturbation in the oxygen-minimum zone offshore Oman:

Bioturbation of modern sediments has been quantified in 2 OMZ regions, on the Peru and Oman margins. In both cases the mixed layer depth, determined by Pb-210 and Th-234 profiles, is much lower within than beneath the OMZ (Smith et al. 2000; Levin et al., in preparation). On the Peru margin, particle mixing rates, measured by Th-234, are reduced within the OMZ (6-13 cm2/y) relative to station beneath the OMZ ( 80-150 cm2/y) (Levin et al. in preparation) No reduction in particle mixing rates was observed within versus beneath the Oman margin OMZ but a longer-lived tracer (Pb-210) was used (Smith et al. 2000).

The mean 210Pb mixed-layer depth for the oxygenated slopes (mean 11.1 cm, s.e. 0.5 cm) was twice that of the Oman slope OMZ (mean 4.6 cm, s.e. 0.2 cm), and this difference was statistically significant (Table 5). In contrast, the mean mixing intensities within the mixed layer were similar for the oxygenated-slope and Oman-slope OMZ data (0.90 and 1.1, respectively, s.e.*0.32 cm; Table 5). These results suggest that bottom-water oxygen levels between 0.1 and 0.3 ml l-1 on the Oman slope yield a decrease in the thickness of the mixed layer, but had no clear impact on the intensity of mixing within this layer.

This information was available prior to the publication of the two Overpeck articles (Anderson et al 2002; Gupta et al 2003). It looks like the claim that “low-oxygen conditions caused by high productivity prevent burrowing organisms from smoothing centennial-scale events” in the Arabian Sea cores was simply made up.

I think this would be an opportune time to ask a good carbonate geologist to assist here. It seems as
as though a good explaination of soil horizons is in order. It is perfectly as it should be to have the youngest materials be the coarsest.

I recall a discussion on CA regarding the varved glacial lake deposits from Iceberg Lake, Alaska. I would suggest that a thourough assessment of the sedimentology of the deposits would be necessary to determine actual dating of the deposits. Location of the site in proximity to turbidite activity could introduce re-worked materials with vastly differing ages from the enclosing sediments.

The McCave biological pump is unlikely to be important in this environment: this is an area with high sedimentation rates, and in an oxygen minimum. Both factors will limit the impact of bioturbation. The biological pump is much more likely to be important areas with a low sedimentation rate, such as that in the figure from Thomson et al 1995, where the entire Holocene is only 30cm long.
From the Arabian Sea site, we can exclude the biological pump as an important factor contributing to the increase in coarse fraction as radiocarbon dates on planktonic foraminifera (part of the coarse fraction) include bomb radiocarbon.

I think that the most likely explanation for the increase in coarse fraction is that the productivity has increased, increasing the flux of forams to the sediment.

RichardT, I’m sorry, but the logic of your posting partially escapes me.
I read the following assertions:
* high sedimentation and oxygen minimum => limited bioturbation impact
* foraminifera <i>include</i> bomb radiocarbon => we can exclude bioturbation as an important factor
* therefore increased coarse fraction is most likely due to increased foram flux
Do I have that correct? If so, then please consider the following alternative explanation that seems equally likely to me, based on the cited references in #14 (Steve M). Again, I may be married to a marine biologist, but I ain’t one.**
* bioturbation exists even in low oxygen environments => Coarse fraction will always be pulled toward surface.
* forams include bomb radiocarbon => as expected, recent flux includes bomb residue
* i.e. coarse fraction at/near sample surface is a mixture of bioturbated material and new flux
* bioturbation, as expected, is always an important factor
My question: what fraction of the coarse fraction contained bomb residue? Is it even possible to measure this? Methinks that would answer our question directly!
MrPete
PS **Yes, we live in Colorado, but dirt-based life has always been a distinct #2 for Leslie, behind marine life. Her greatest joy is mucking about on the cold ocean floor off the Monterey (CA) coast. 🙂 [Oh, and 100% of the above early-morning ramblings are my own.]

The McCave biological pump is unlikely to be important in this environment: this is an area with high sedimentation rates, and in an oxygen minimum. Both factors will limit the impact of bioturbation.

While this seems plausible, surely this is only a hypothesis and is contradicted by the direct observations in the Arabian Sea Oxygen-Minimum Zone in Smith et al 2000 mentioned in the comment above. Another comment from Smith et al :

Brief descriptions of X-radiographs from each core are presented in Table 6, and representative X-radiographs are presented in Fig. 5. Sediments from the 400-m station were characterized by fine vertical burrows in the top 2-3 cm, as well as faint centimeter-scale laminations within this zone; below ~3 cm, sediments were quite homogeneous. Sediments from the 700-m station contained larger, vertical tubes within the top 3-7 cm; many of these were ampeliscid amphipod tubes that protruded through the sediment-water interface. Above a sediment depth of 4-8 cm, the only visible structure was biogenic, while at depths greater than 4-8 cm, sediments often increased abruptly in density and exhibited no bioturbation; this density shift corresponded roughly to the mixed-layer depth for 210Pb profiles from this station (Fig. 3). At the 850-m station, the top 2-4 cm of sediment were riddled with burrows yielding a very low-density layer; numerous cirratulid mudballs protruded above the mean sediment}water interface. Below 4 cm, sediment density increased but remained heavily bioturbated.

This reads to me as direct evidence of bioturbation in the Oman Margin OMZ regardless of any theoretical reasons for the non-existence of the phenomenon.

#22
If you want perfectly undisturbed sediments then you need to work in an anoxic basin like Cariaco. Such locations are rare, so we have to compromise and accept some bioturbation, but selecting sites where its importance is likely to be minimal. The sea off Oman is one such location, where rapid sedimentation rates and low oxygen concentration should limit bioturbation.
Smith et al use Pb210 to determine the mixing layer thickness, and conclude that

the mixed-layer depth within the Oman OMZ was only half the global mean for bioturbated sediments (9.8 cm) reported by Boudreau (1994). Thus, we conclude that the mixed layer is substantially reduced in the Arabian Sea OMZ

Since the sedimentation rate, ~100mm/ka, is much above the average for the ocean, bioturbation is minimal here.
The site is not free from bioturbation, as the authors acknowledge

…it comes from the minimum in G. bulloides abundance,where mixing could…
…
mixing indicated by the bomb radiocarbon may have smoothed the signal, incompletely mixing sediment over several millimeters (several decades).

But for the deep ocean, this is good site for high-resolution work.

#21
The radiocarbon dates are in table S1. Its quite easy to develop scenarios and test if they are consistent with the 14C dates

Richard T, thanks for the very helpful remarks. I can understand the reason for saying that bioturbation would be less at this site than many other sites and I agree that there are sentences in this article that indicate some possible bioturbation, but the sentence that I quoted above:

The bioturbation smoothing that would normally affect fine-scale variability is minimal at this site owing to the strong oxygen-minimum zone that exists in the Oman margin.

does seem to me to go a bridge too far when it comes to mm-scale detection. But I see where you’re coming from. Here’s a plot representing my understanding of the RC2730 data. The solid points are RC age measurements and depth midpoints (2 mm) intervals. The depth scale is cm. If one takes as a modern point a reservoir age of 604 years (marked in red), then one gets the depth-RC age line shown in a dashed line. The four RC age measurements in the top 1 cm are all too young (one measurement is in the 0.8-1.0 cm depth). The RC age measurement at 3 cm is perhaps unaffected to date by bomb radiocarbon – although if the mixing depth is 4.5 cm as indicated in Smith et al, perhaps there is still some opportunity for this value to change in the future with ongoing bioturbation. However, blue dashed line indicates potential bioturbation based on the information available at present, assuming no bioturbation will take place at the 3 cm measurement. Perhaps the line is a bit steeper, perhaps a bit shallower, but it’s as plausible as anything else on the info.

As I understand, the bioturbation takes place primarily by organisms moving fines downward and thus the coarse fraction is percolated upward. On the basis that the blue dashed line represents fines going downward in some fashion, the coarse fraction will presumably have an older RC age than the reservoir age – I’ve plotted a dotted line to represent this – I haven’t tried to calculate a materials balance – I’ve just plugged a value on the y-axis to show a potential effect since it seems to me that there will have to be some effect.

Now the increase in coarse fraction takes place entirely in the top 1.2 cm, where there is evidence of bioturbation. Quantitatively, it could very well be that the amount of bioturbation needed to boil up enough coarse fraction can’t account for the observed increase in coarse fraction. That’s one thing. But right now it seems to me that Anderson et al have simply arm-waved through the matter; surely some sort of calculation is needed to show that the increase in coarse fraction could not have arisen from bioturbation or some other mechanical effect. Otherwise, in a first look at this, in the language of paleoclimate, there’s a “remarkable similarity” between the indicated scale of bioturbation and the indicated scale of attenuation of coarse fraction.

A prediction:
If bioturbation-driven sediment sorting (rather than homogenisation) is important, it should be a general pattern, observable in most cores.
Whereas, if the increase in coarse fraction is climatically driven, it should only occur in a few sites.
%coarse fraction is measured fairly often, so it should be possible to test your hypothesis.
#24 CI?

#26. Doesn’t the answer already exist: McCave 1988 and the subsequent literature describe a biological pumping upward of coarse fraction, which Thomson et al 1995 says leads to a radiocarbon offset in the coarse fraction. I presume that McCave, Thomson etc based their description of biological pumping of coarse fraction on a number of cores; I haven’t seen any articles arguing that the issue was one of limited application. Indeed bioturbation seems to be a general concern – otherwise why would Anderson et al have even mentioned it, even to dismiss it.

In the past, I’ve trawled through ocean literature looking for very high resolution sites. I don’t recall seeing any other cores of a similar accumulation rate in which the coarse fraction was sampled at 0.2 mm intervals in the top 1.3 cm and where the authors have satisfied themselves that they have the top of the core intact. Suggestions welcomed.

We’re not talking about an ad hoc interpretation in the Arabian Sea – indeed it was this literature that prompted the suggestion. In fact, it’s Anderson et al who have the ad hoc interpretation. They suggested that the Arabian Sea would be an exception to this pattern (arm waving through the argument without a reference), but I don’t see that the conditions necessary to create an exception have been established.

Here’s a simple calculus exercise for someone – I could probably have done it in a few minutes when I was 19 but alas I’m not 19 any more. Let’s round the accumulation rate to 10 cm/kyr or 1 cm/100 years. Our sample was taken in 1985 and let’s suppose that the bomb radiocarbon was introduced 30 years earlier starting in say 1955.

Let’s now assume that post-1955 material has been bioturbed downwards in a sufficient amount to yield the line shown in #25 above (or some exponential that looks like it). Let’s assume that the downward going material is all fines and that the upward moving material is coarse – as a first approximation.

This should yield some sort of exponential looking function for coarse fraction. And there should be enough information in this to calculate the coarse fraction curve. Any takers?

One of the advantages of working in a university is that you get to go to interesting talks. Today I listened to a presentation on organic carbon geochemistry in the Arabian Sea. A coincidence as its the present hot topic here. As has been pointed out the Arabian Sea is an area of high productivity with organic carbon rich sediments, particularly around its margin. There is a strongly developed oxygen minima in the water column. However, the surprising feature of the talk, at least to my mind was the significant benthic activity of micro- and macrofauna. Using 13C labelled carbon they identified very rapid overturning of primary productivity carbon in the substrate with the carbon being respired as CO2, and taken up in carbohydrates, lipids etc. of the fauna. As I understood the talk many of the species are specially adapted with high haemoglobin levels enabling them to function in the oxygen minima. Bioturbation was clearly evident in some of the core material. The site was different to that being discussed here being towards the eastern margins of the basin with influence from the Indus.

He shows a very large proportion of fines moving downward in an empirical situation, I don’t know what proportion of coarse is being pumped upward, but I suspect that parameters required to yield a pronounced uptick of coarse in the top 5-10 mm may not be implausible,

#33
The biological pump is certainly a potential problem when comparing the fine and coarse fraction from the same core, especially in low accumulation sites. But in the Arabian Sea study, all the proxy investigated, and the dates are both on the coarse fraction.
The forams will probably have been counted in the >150 micron fraction. You have invoked a process that could separate the coarse fraction from the fines, but you need a process sensitive enough to differentiate between the different species of foram, all in the >150 fraction, by size. We also know that there has been downward bioturbation of forams, because of the bomb-contaminated dates. One of these contaminated dates (at 4-6cm) is on pure bulloides – not exactly consistent with the biological pump hypothesis.

#34. Your point about downward perturbation of large particles of G Bulloides is interesting – I hadn’t thought about that. But I don’t see that it resolves anything – it might even complicate things.

I’m presuming that the mechanism of bioturbation and the upward movement of coarse particles relative to downward movement of fines is something that is relatively well-established in the literature – I’m not presenting this as my hypothesis; I’ not trying to prove the effect in general – I’m assuming that’s already been done. I’m only trying to weight Anderson’s cursory dismissal of the process re RC2730.

You observe that the 4-6 mm depth sample was taken from G Bulloides, and posit plausibly that this would have been taken from the plus 150 micron fraction.

Here are a few thoughts on what that might imply.

First, is it possible that the sample was taken on G Bulloides fines if there are such things? You’re hypothesizing that the test would be done on coarse, but I don’t think that they actually say that – plausible at it seems.

Second, it seems to me that I’ve read that fines are often entrained with forams. Isn’t it possible that washing would not fully separate the fines and that it is the entrained fines that are showing the bomb effect?

Third, let’s suppose that post-1955 coarse G Bulloides have been bioturbed downward. If there’s measurable bioturbation of large particles, that suggests a very active bioturbation which would affect fines even more strongly – something that seems conclusively proven by Wheatcroft’s experiments. A more active bioturbation would indicate to me a heightened probability of biological upward pumping, notwithstanding the downward movement of some coarse particles.

Fourth, if the coarse G Bulloides are in situ, then the accumulation rates would have to be higher than are being used.

So there’s still a problem under any of these situations.

You’ve also misconstrued the nature of the effect that I had posited in connection with the inter-relationship of the G Bulloides series relative to the coarse fraction series. I wasn’t thinking about upward pumping leading to a concentration of G Bulloides relative to G Ruber (although if there are size differences or dissolution differences, that would be worth examining – it seems to me that I’ve read that G Ruber is relatively fragile and thus would presumably be more prone to fragment.) The point that I had in mind was that biological pumping would blur the coarse fraction so that the coarse in the mixed layer would be an integral of coarse over the past 450 years or so. Anderson 2001 discusses the problems of convolving a signal and I don’t see why the same argument wouldn’t apply here.

Yes, we need the input of a good carbonate geochemist. I was only a generalist, so I offer a general comment that I have not researched – yet. Please tell me if I should or if the answer is known.

In a column where there is simultaneous dissolution of old carbonate material and formation of new material, in strata where the conditions permit each, is there is a possibility of isotope homogenisation? That is, can the new shell isotope ratios be perturbed because some of their building block material comes from older, dissolving shells with different isotope ratios? The answer might well be that the atmosphere supplies adequate new carbon and oxygen, with whatever isotope signals are in the air at the time, but then one has to account for the calcium as well. Is it mobile up the column or does it come from airborne dust or from lateral transport? I simply don’t know the relative likelihood of these possible contributions.

I have similar conceptual problems with corals, as we have such large reefs here. We have coral of many ages decaying and producing CO2 (I presume) and thus at least some mixing of isotopes of both oxygen and carbon. How much, I do not know.

Richard T, I was able to simulate the following profile for coarse fraction with a simple simulation of a bioturbation mechanism of a steady state deposition of 0.9 fines and 0.1 coarse. I could play with the parameters a little more and get an even closer match to the observed curve. This was my first run with the mechanism described below (although I experimented with a couple of other mechanisms).

The premises of the simulation were as follows:
1) all bioturbation activity originated at the surface layer in the current year, the depth of secretion had a negative exponential shape and all downward secretion were fines;
2) upward percolation balanced the bioturbation and all upward percolation was in coarse. (If upward percolation is partly fines, then I’m pretty sure that somewhat different parameters could be found to yield any shape achievable with the method here.)
3) averages were taken over 10 year intervals and plotted.

The negative exponential shape for bioturbation secretion is attested in Wheatcroft; the coarse fraction percolation in McCave; Thomson et al etc. Bioturbation is attested in the Oman OMZ, despite arm-waving by Overpeck’s associates.

This doesn’t prove that the coarse fraction profile was generated by bioturbation. However the coarse fraction profile really has an alarmingly simple shape and the fact that you can generate that shape with a simple implementation of a bioturbation mechanism should really give people some pause.

You observe that this doesn’t prove that the G Bulloides percentage are affected by this phenomenon. However, the correlation between G Bulloides percentage and coarse fraction percentage is 0.91 – a very high percentage in paleoclimate; the closing portion of the G Bulloides profile is also alarmingly untextured and this suggests to me that some nonclimatic mechanism may be involved – perhaps allied to the concentration occurring in the coarse fraction or perhaps some independent mechanism operating in a similar way.

What should concern general readers aside from this is the fact that the percentage G Bulloides result of Overpeck and associates is essentially a one-off result, which has nonetheless been inhaled into multiproxy studies (Moberg, Juckes) as though high confidence could be placed in the original study. What do we really know about G Bulloides percentages? Is there a valid relationship to temperature? The only study containing detailed information on G Bulloides over the past millennium is by Black at Cariaco and I’ll look at that in the next few days.

#38
Please can you post the code for your simulation – or better still modify if to calculate the mean age of the coarse fraction in each 1mm slice. I propose to demonstrate that your simulation is inconsistent with the radiocarbon dates, and hence that the mechanism you propose is inadequate.

#39. I doubt that this particular mechanism will make any difference to radiocarbon date. Under the mechanism shown here, the coarse fraction only percolates upward a few years, but that’s sufficient to create an inventory in the mixed layer.

HAving said that, this particular mechanism would not result in huge differences in age either – for that to happen, some different mechanism would have to exist. To accomplish a substantial modification of coarse fraction age, I’d have to think up different sorts of parameters – and I’ve not explored this.

The “remarkable similarity” in the increase in coarse fraction in the mixed layer and the increase in G Bulloides percentage in the mixed layer suggests to me that some sort of concentrating mechanism might well be working whether or not I’m in a position to identify precisely what it is. As I understand it, the only other somewhat comparable data set is the Black Cariaco series, which has a completely different look to it in the top portion.

Without some replication, it seems incredibly amateur to incorporate such trial balloons into multiproxy reconstructions.

#41. That’s just the starting guess working itself out of the simulation. Try re-doing the simulation using as a starting point, the values from the 500th iteration. I just did this. The effect that you’re worrying about doesn’t occur then. This sort of effect is not unusual when you make a starting guess without knowing the steady state value. That’s why I used the 500th simulation. Not a problem. (Not to say that there aren’t other issues, this is rough and ready, but that’s not one of them.)

PS. You can start with the steady state by simply doing 1000 iterations and then chopping off the first half. I’ve edited the script a little to do this.

The remarkable similarity in the increase in coarse fraction in the mixed layer and the increase in G Bulloides percentage in the mixed layer suggests to me that some sort of concentrating mechanism might well be working whether or not Im in a position to identify precisely what it is.

Actually a common mechanism is very easy to hypothesize. Suppose that the distribution of G Bulloides and G Ruber in the plus 150-micron size is coarser for G Bulloides. For example, suppose than only 10% of plus 150-micron G Ruber are plus 250-micron, while say 30% of G Bulloides are. (And it’s my understanding that the hypothesis of more large G Bulloides is not an idle hypothesis as I’m pretty sure that I’ve read somewhere that there are more large G Bulloides than G Ruber.)

Then precisely the same mechanism that generates the increased coarse fraction in the top 1.2 cm mixed layer would also generate the increased percentage of G Bulloides. Given that there’s a 0.9 correlation between pct G Bullolides and pct coarse fraction, the idea of a common mechanism is rather neat.

I don’t know what percentage of the total coarse is made up of G Bulloides – I presume that there are other constituents in the coarse fraction besides G Bulloides and that there is an increase in other coarse components besides G Bulloides: maybe you could give some estimates of what sort of distributions one would encounter.

It looks more and more to me like the pct G Bulloides series contains a spurious effect which then introduces a spurious regression with NH temperature in classic Team style. If so, it will be a rather nifty example as the series is a major contributor to the Moberg and Juckes composites – I think that it was the largest contributor to modern-medieval differences in Moberg. So it would be rather fun if it proved to be another completely spurious regression.